Why would Rockstar give up that 18% extra when they know its going to sell just as well on either store? They've expressed their disdain at the way Valves run things before, all the extra systems required to release on their platform, I wouldn't be surprised if this wasn't on Steam now developers have choice on the matter.
The user base is now essentially equal in size, but one store offers to work closer with you, allow better integration, and gives you 25% more money from your sales. The only one who'd have to bribe here is Valve surely?
It's worth noting, Epic "exclusives" doesn't mean its only sold on the Epic store. These exclusives are done in a coalition of more or less all the PC gaming stores atm with heavy integration with developers own stores. It's literally, just not Steam, because its problematic.